A normal human ear transmits sounds as shown in FIG. 1 through the outer ear 101 to the tympanic membrane 102 which moves the bones of the middle ear 103 that vibrate the oval window and round window openings of the cochlea 104. The cochlea 104 is a long narrow duct wound spirally about its axis for approximately two and a half turns. It includes an upper channel known as the scala vestibuli and a lower channel known as the scala tympani, which are connected by the cochlear duct. The cochlea 104 forms an upright spiraling cone with a center called the modiolar where the spiral ganglion cells of the acoustic nerve 113 reside. In response to received sounds transmitted by the middle ear 103, the fluid-filled cochlea 104 functions as a transducer to generate electric pulses which are transmitted to the cochlear nerve 113, and ultimately to the brain.
Hearing is impaired when there are problems in the ability to transduce external sounds into meaningful action potentials along the neural substrate of the cochlea 104. To improve impaired hearing, auditory prostheses have been developed. For example, when the impairment is related to operation of the middle ear 103, a conventional hearing aid may be used to provide acoustic-mechanical stimulation to the auditory system in the form of amplified sound. Or when the impairment is associated with the cochlea 104, a cochlear implant with an implanted electrode can electrically stimulate auditory nerve tissue with small currents delivered by multiple electrode contacts distributed along the electrode. Although the following discussion is specific to cochlear implants, some hearing impaired persons are better served when the stimulation electrode is implanted in other anatomical structures. Thus auditory implant systems include brainstem implants, middle brain implants, etc. each stimulating a specific auditory target in the hearing system.
FIG. 1 also shows some components of a typical cochlear implant system where an external microphone provides an audio signal input to an external implant processor 111 in which various signal processing schemes can be implemented. For example, signal processing approaches that are well-known in the field of cochlear implants include continuous interleaved sampling (CIS) digital signal processing, channel specific sampling sequences (CSSS) digital signal processing (as described in U.S. Pat. No. 6,348,070, incorporated herein by reference), spectral peak (SPEAK) digital signal processing, fine structure processing (FSP) and compressed analog (CA) signal processing.
The processed signal is then converted into a digital data format for transmission by external transmitter coil 107 into the implant stimulator 108. Besides receiving the processed audio information, the implant stimulator 108 also performs additional signal processing such as error correction, pulse formation, etc., and produces a stimulation pattern (based on the extracted audio information) that is sent through an electrode lead 109 to an implanted electrode array 110. Typically, this electrode array 110 includes multiple electrode contacts 112 on its surface that provide selective stimulation of the cochlea 104.
FIG. 2 shows various functional blocks in a typical CI signal processing system using the CIS stimulation strategy. An audio input pre-processor 201 includes a pre-emphasis filter 203 that receives an input audio signal from a microphone and attenuates strong frequency components in the audio signal below about 1.2 kHz. FIG. 3 shows a typical example of a short time period of an input audio signal from a microphone. The sound pre-processor 201 also includes multiple band pass filters (BPFs) 204 that decompose the audio signal from the pre-emphasis filter 203 into multiple spectral band pass signals as shown, for example, in FIG. 4. As shown in FIG. 5, each band pass signal 501 is thought of as having a fine structure component 502 and an envelope component 503 (typically derived by Hilbert transformation). The filtered envelope signal 504 oscillates around the zero reference axis line with a frequency that is related to the fundamental frequency F0 of the band pass filter.
A sound processor 202 includes envelope detectors 205 that extract the slowly-varying band pass envelope components of the band pass signals, for example, by full-wave rectification and low pass filtering. The sound processor 202 also includes a non-linear (e.g., logarithmic) mapping module 206 that performs compression of the envelopes to fit the patient's perceptual characteristics, and the compressed envelope signals are then multiplied with carrier waveforms by modulators 207 to produce electrode stimulation signals in the specific form of non-overlapping biphasic output pulses for each of the stimulation contacts (EL-1 to EL-n) in the electrode array that is implanted in the cochlea 104 reflecting the tonotopic neural response of the cochlea.
CIS stimulation imposes a fixed stimulation rate on the electrical pulses that form the electrode stimulation signals and therefore cannot represent periodicity components of the input audio signal. On the other hand, FSP stimulation (and its variants) does represent the inherent periodicity of sensed audio signals. FSP generates electrode stimulation signals using patterns of stimulation pulse trains responsive to detection of specific pre-defined band pass components such as zero crossing events. In FSP, CSSS sequences are applied at zero crossings of the fine structure components. The CSSS sequences can transmit information on instantaneous frequency up to patient-specific limits.
U.S. Pat. No. 7,920,923 to Laback describes adding phase jitter to the stimulation signal pulses to improve perception of interaural time difference (ITD) information. Binaural audio signals are generated that represent sound associated with a user's left and right ears respectively. Based on the binaural audio signals, corresponding binaural stimulation signals are generated for electrical stimulation of auditory nerve tissue of the user, where the binaural stimulation signals include fine structure components with periodic characteristics and ITD information. A phase jitter component is added to the binaural stimulation signals to reduce the periodic characteristics of the fine structure component while preserving the ITD information between the left and right ears. Since binaural adaptation is a phenomenon which occurs for periodic signals, introducing artificial phase jitter into the stimulation signals reduces the periodicities of the signals to make the listener less prone to binaural adaptation. The artificial phase jitter is based on the fine structure component.
While Laback showed that adding phase jitter improves hearing directionality, Hancock et al., Neural ITD Coding with Bilateral Cochlear Implants: Effect of Binaurally Coherent Jitter, J Neurophysiol., 108(3), 2012 Aug. 1, p. 714-728 (incorporated herein by reference) found that this improvement is mainly due to the use of short inter-pulse intervals. When auditory neurons are electrically stimulated, their responses are in phase with the electrical stimuli up to a patient-specific stimulation rate. Hancock reported that the neural response to constant-rate pulse trains in the inferior colliculus of cats is an on-going response that is phase-locked to stimuli of up to 320 pps. Above the 320 pps stimulation rate limit, the auditory neurons fire only in response to the onset of pulse bursts. Hancock further showed that introducing short inter-pulse intervals with random phase-jitter significantly changed the neural response in the inferior colliculus for stimulation rates above 320 pps in sustaining the firing a significant number of neurons. But introducing random phase jitter may have detrimental effects on other auditory functions, for example rate pitch perception, and speech understanding potentially may suffer. And current auditory implant systems have no way to extend the range of phase-locking of neural responses to stimulation pulses.